1. Field of the Invention
The present invention relates to a display device for displaying an image by using liquid, particularly to a display device having a memory effect.
2. Description of the Related Art
Currently, printed matter is widely used for the purpose of advertisements etc. Although printed matter cannot be visually identified when there is no lighting, they can provide a full color printing and the like without consuming electric power etc. They also can be produced in a large volume and are inexpensive. However, when the printing content is frequently changed, it is difficult to cope with frequent changes in a short period of time since the period from a printing step to a display step is long.
To solve this, display devices having a low power consumption have been explored. One example of this includes a display device using electrophoresis. However, this display device has difficulty displaying color, although it can display a black and white image. When displaying an image in color, a technique of adding a color-filter array substrate to the above mentioned display device to display a color, or of laminating a plurality of display devices which produce a different color is adopted. However, when using a color-filter array substrate, a problem arises in which white display luminance is lowered, and it is not possible to obtain a display screen having a sufficient brightness for a full color display etc. On the other hand, when a plurality of display devices are laminated, a problem arises in that the structure becomes complicated and the number of display devices increases thereby becoming expensive.
A display device for overcoming the above described limitation for colorization has been proposed in Japanese Patent Laid-Open No. 2005-84166. Referring to FIG. 1, the operation of this display device will be described. The display device is generally made up of image display plate 72, and a unit for forming segmented fluid stream (hereinafter, designated as a segmented fluid stream forming unit) 71. In segmented fluid stream forming unit 71, a plurality of colored fluids are mixed to obtain desired color liquid. By alternately discharging the desired color liquid and separating fluid from segmented fluid stream forming unit 71 to a flow passage, a segmented fluid stream is formed. Image display plate 71 has one winding flow passage 1. In this flow passage 1, a segmented fluid stream is moved by the discharge operation of segmented fluid stream forming unit 71 and, when it reaches the desired display position, the discharge operation is stopped. In this way, it becomes possible to form a desired image on image display plate 72.
However, the display device disclosed in Japanese Patent Laid-Open No. 2005-84166 has a problem described below.
According to paragraph [0034] of Japanese patent Laid-Open No. 2005-84166, for separating fluid 73 shown in FIG. 2, a liquid which has no compatibility with liquid 4 which serves to provide a display is recommended. When this liquid stream is pressurized by a pump to move to a desired position thereby providing a predetermined display image, it is essential to insert the above described separating fluid to avoid the mixing of liquids which carry pixel information (hereinafter, referred to as an “pixel information liquid stream”). However, because of the pumping pressurization from behind, it cannot be expected that the fluid stream will be uniformly pressurized in its sectional direction. As the result, as the liquid continues to move in pressurizing direction 74, mixing between pixel information liquids takes place as shown in FIG. 2. This mixing will become a more severe problem when the flow passage in an image display plate is long.
According to paragraph [0034] of Japanese Patent Laid-Open No. 2005-84166, the separating fluid may be a gas. However, since gas is compressed when it is pressurized, its volume is easily changed. Therefore, to avoid mixing between pixel information liquids, the use of a gas is not preferable.
As so far described, according to the technique disclosed in Japanese Patent Laid-Open No. 2005-84166, it is difficult to move fluid in a stream, in which pixel information liquid and separating fluid are alternately disposed (hereinafter, designated as a “pixel information liquid stream”), in a stable condition.
Then, it may be expected that providing a large amount of separating fluid and a large interval between pixel information liquids can prevent the mixing of a pixel information liquid stream.
However, in this case, since the volume of the separating fluid increases, the volume ratio of the pixel information liquid on the image display plate will decrease. Therefore, the area ratio of the pixel region to be used for display will decrease, thereby causing a problem that the displayed image becomes dark.